1. Field of the Invention
The present invention relates to a vehicular differential limiting mechanism and particularly to a differential limiting mechanism using a compound type planetary gear unit for generating a differential limiting torque which is proportional to an input torque.
2. Prior Arts
When a two wheel drive vehicle incorporating a conventional differential travels in such a state that one of left and right drive wheels is on a road surface with a low friction coefficient and the other is on a road surface with a normal friction coefficient, the vehicle may exhibit an unstable behavior because the wheel on the road surface with a low friction coefficient slips and the other wheel on the road surface with a normal friction coefficient loses traction, too. There occurs a similar phenomenon when the vehicle travels on bumpy roads or traverses steep grades. When the vehicle travels on bumpy roads, the vehicle may experience frequently such a state that one of the left and right drive wheels is detached from a road surface and a drive wheel on the opposite side loses traction. Further, when the vehicle traverses a steep grade, a drive wheel in the high position reduces traction and resultantly a drive wheel in the low position also reduces traction.
In general, differential limiting apparatuses capable of generating a differential limiting torque proportional to input torque are of much benefit to improve these phenomena and to enhance vehicle maneuverability under adverse road conditions. Miscellaneous types of differential limiting apparatuses have been developed to date and some of them have been put into practical use.
Japanese patent application laid-open No. Toku-Kai-Hei 5-280597 discloses a differential limiting apparatus which comprises a first side gear connected to one output shaft, a second side gear connected to the other output shaft, a first pinion having a first gearing section at one end thereof which meshes with the first sun gear and having a second gearing section at the other end thereof, a second pinion having a third gearing section at one end thereof which meshes with the first gearing section and having a fourth gearing section at the other end thereof which meshes with the second gearing section and at the same time with the second side gear, and a housing having a plurality of holes for accommodating and rotatably supporting a pair of the first and second pinions therein, so as to produce a differential limiting torque by friction forces generated in the meshing portions of gears and the sliding portions between the pinions and the housing or between the pinions and the holes.
Further, as another example of disclosures, there is a differential limiting apparatus proposed in Japanese patent application laid-open No. Toku-Kai-Hei 4-271926. The differential according to this disclosure includes a pair of side gears (worm gears) whose respective end is connected with drive axle ends respectively, a plurality of pairs of element gears (worm wheels) meshing with side gears and at the same time meshing with each other at respective gearing sections provided at both ends thereof, a housing for accommodating and supporting the element gears, and bearing members such as thrust washers provided between the ends of the element gears and the housing, so as to generate a differential limiting torque proportional to an input torque.
However, the differential limiting apparatus according to the former prior art has several problems coming from using a lot of gears constituting that apparatus. That is, a first problem is a deviation of the differential limiting torque which is caused by many places where friction originates.
Secondly, there is a problem of backlash in gears. So many gears produce an increase of accumulated backlashes which leads to play in the power transmission system such as a play between left and rear wheels or between a propeller shaft and the left or right wheel, resulting in an adverse effect on vibrations or noises. Especially when a driver repeats depressing and releasing the accelerator pedal or when he suddenly depresses the accelerator pedal from the release condition, strange noises or vibrations tend to occur in the power transmission system of the vehicle. Reducing backlashes of gears is not acceptable from the view point of an increase in manufacturing cost due to high precision machining processes on the housing and gearing components.
Further, another problem is a small degree of freedom in setting gear specifications due to the condition of the number of teeth for arranging a plurality of pinions at an even angular increment. Also, a further problem is a limitation in numbers of teeth of pinions due to a necessity for arranging a plurality of pinions at an even angular increment. Small numbers of teeth incur weak strength of gears.
On the other hand, in case of the differential limiting apparatus using the combination of worm gears and worm wheels, as described in the latter case of the prior art, a separate lubricating system is needed to be incorporated because lubrication oil to lubricate these worm gear and worm wheels must be a hypoid gear oil containing extreme-pressure additives. The separate lubricating system is disadvantageous in employing an automatic transmission oil (ATF) for both the automatic transmission and the differential, particularly in case where the automatic transmission and the differential are accommodated in a common housing.
The case is similar to the differential using a combination of side gears and pinions as described in the former case of the prior art.